This invention relates to a process for producing methacrylic acid. More particularly, the invention relates to a process for producing methacrylic acid at high yield, through vapor phase catalytic oxidation of methacrolein.
A large number of proposals have been made in the past concerning the catalysts useful in preparation of methacrylic acid through oxidation of methacrolein with molecular oxygen or molecular oxygen-containing gas, in the presence of an oxidation catalyst. Demands for improvements in activity, selectivity and life of the catalyst, however, have still long been pending.
The object of the present invention is to provide a process which enables production of methacrylic acid through vapor phase catalytic oxidation of methacrolein, at high yield and with stability for a prolonged period.
The reaction to produce methacrylic acid by vapor phase catalytic oxidation of methacrolein using fixed bed shell-and-tube reactor is apt to give rise to such undesirable side reactions as parallel reactions or sequential reactions which reduce selectivity for methacrylic acid and in consequence reduce its yield. The side reactions also provide causes for transfer of used catalyst to undesirable oxidation-reduction condition and clogging of pores in the catalyst, leading to shortening of catalyst life.
We have made concentrative studies how to effectively inhibit such undesirable side-reactions and now discovered that the inhibition could be achieved to improve methacrylic acid yield, by dividing the catalyst layer in each reaction tube into at least two layers to provide plural reaction zones, and filling the catalyst in the reaction zones in such a manner that the amount of the catalytically active component per unit volume of the reaction tube decreases from the gas inlet portion toward the gas outlet portion; and also that whereby catalyst deterioration can be inhibited to allow stable methacrylic acid production at high yield over prolonged period.
Thus, according to the present invention a process for producing methacrylic acid through vapor phase oxidation of methacrolein with molecular oxygen or a molecular oxygen-containing gas sing a fixed bed shell-and-tube reactor filled with catalyst is provided, which is characterized in that
(1) the catalyst layer in each reaction tube is divided into at least two layers in the axial direction of the tube to provide plural reaction zones, and
(2) the catalyst is filled in the plural reaction zones in such a manner that the amount of the catalytically active component per unit volume of the reaction tube decreases from the gas inlet portion toward the gas outlet portion.
According to the invention, the catalyst layer in each reaction tube in a fixed bed shell-and-tube reactor to be used in the process is divided into at least two layers to provide plural reaction zones. The more the number of reaction zones, the more effectively prevented are the side reactions, but from industrial standpoint it is economical to provide 2 to 3 reaction zones. The dividing ratio of the reaction zones is variable depending on composition and shape of catalyst to be filled in the reaction zones and cannot be unqualifiedly specified. It can be suitably determined in each individual occasion so as to secure the optimum activity and selectivity as a whole.
According to the present invention, catalyst is filled in the plural reaction zones in such a manner that the amount of the catalytically active component per unit volume of each reaction tube decreases from the gas inlet portion toward the gas outlet portion.
As the catalyst useful in the present invention complex oxides which are expressed by the following general formula (1) and which contain molybdophosphoric acid are conveniently used:
MoaPbAcBdCeOxxe2x80x83xe2x80x83(I)
wherein Mo is molybdenum; P is phosphorus; A is at least an element selected from the group consisting of arsenic, antimony, germanium, bismuth, zirconium, selenium, cerium, copper, iron, chromium, nickel, manganese, cobalt, tin, silver, zinc, palladium, rhodium and tellurium; B is at least an element selected from the group consisting of vanadium, tungsten and niobium; C is at least an element selected from the group consisting of alkali metals, alkaline earth metals and thallium; and O is oxygen; and a, b, c, d, e and x represent atomic ratios of Mo, P, A, B, C and O, respectively, where when a is 12, b is 0.5-4, c is 0.001-5, d is 0.001-4 and e is 0.001-4 and x is a numerical value determined by degree of oxidation of each of the elements.
Preparation methods and starting materials of the catalyst to be used in the present invention are not critical, but any of the methods and starting materials heretofore generally employed for preparation of this type of catalyst may be used.
Form of use of the catalyst in the present invention again is subject to no critical limitation. Molded catalysts obtained by molding the complex oxide which is expressed by the earlier given general formula (I) and which contains molybdophosphoric acid by any of conventionally practiced methods, such as extrusion molding, tabletting or the like; or carrier-supported catalysts in which the complex oxide is carried on conventionally used carriers such as silicon carbide, xcex1-alumina, silica-alumina, zirconium oxide, titanium dioxide and the like, can be used. Where a carrier-supported catalyst is used, the amount of catalytically active component signifies the weight of the catalyst from which the weight of the carrier is subtracted.
Shape of the catalyst to be used in the present invention again is subject to no critical limitation. Shapes suitable for individual occasion, such as pellets, spheres, rings, tablets and the like can be suitably selected.
Examples of typical means for filling the catalyst in the reaction zones to achieve the reduction in the amount of the catalytically active component per unit volume of the reaction tube from the gas inlet portion toward the gas outlet portion include the following:
{circumflex over (1)} change shape of molded catalyst, e.g., fill the gas inlet portion with pelletized catalyst, and the gas outlet portion, with ring-formed catalyst;
{circumflex over (2)} dilute molded catalyst with inert carrier, e.g., fill the gas inlet portion with undiluted catalyst, and the gas outlet portion, with a mixture of the catalyst with inert carrier;
{circumflex over (3)} change specific gravity of molded catalyst, e.g., fill the gas inlet portion with catalyst of greater specific gravity, and the gas outlet portion, with the catalyst of less specific gravity which can be readily obtained, for example, by increasing the amount of water used for the preparation of the catalyst;
{circumflex over (4)} use molded catalyst in combination with carrier-supported catalyst, e.g., fill the gas inlet portion with a molded catalyst and the gas outlet portion, with a carrier-supported catalyst;
{circumflex over (5)} change the amount of catalyst carried on support, e.g., fill the gas inlet portion with a support carrying greater amount of the catalytic component and the gas outlet portion, with that carrying less amount of the catalytic component.
Operation conditions of the vapor phase catalytic oxidation reaction according to the present invention are not critical, but those generally used may be adopted. For example, the vapor phase oxidation reaction can be carried out by introducing a starting gas containing 1-10 volume % of methacrolein, 3-20 volume % of molecular oxygen, 0-60 volume % of steam and 20-80 volume % of inert gas such as nitrogen, carbon dioxide, etc. into the catalyst layers in each reaction tube, and reacting them under such conditions as at temperature ranging from 250-450xc2x0 C., pressure of 1-10 atmospheres and space velocity (SV) of 300-5000 hrxe2x88x921.
According to the process of the present invention, undesirable side reactions can be effectively inhibited and object methacrylic acid can be produced at high yield.
The inhibition of side reactions also results in reduced catalyst degradation and extension in catalyst life. Thus, object methacrylic acid can be produced at high yield stably over prolonged period.